That would be pretty funny ha....you get to the pier and the ship is gone. The way I see it you have a few options. You could swim....take a cab...borrow a burro from one of the locals and ask for directions, or rent a Jet Ski and pull a James Bond to rendevous with the ship a few miles out.

Dan in Winnipeg, you are a riot!
But seriously folks, you really have just two options; if the ship is not too far out you can get a water taxi to take you out. This is extremely expensive and will not make you very popular with the Bridge Officers; or you can try to arrange transportation to the next port--also very expensive.
I can't imagine doing it on purpose--but if you did, your options are still the same--or you can arrange transportation home. Just be sure you have your passport with you.

We actually cruised with a couple (and almost went with them) that missed a reboard at Gibralter and missed it again in Barcelona when our ship was denied access to the port (because we had just been to Gibraltar -- British/Spanish politics, you know). Never learned what happened to them

We met them on an NCL cruise from Dover to Rome. The woman's Dad had been some sort of diplomat in Morroco and she and her husband wanted to do a day-trip there from Gibralter. Apparently, the political dispute between UK and Spain led to delays at the border as they tried to return to the ship from Spanish-controlled territory.

Wouldn't it make more sense to just travel direct to the country that you want to visit, vs. taking a cruise and then disappearing from the ship? Seems like this would be much less a hassle. One might think you would be using the ship to gain entrance to a port without authorization.